1. Field of the Invention
The present invention relates to a group III nitride semiconductor crystal substrate and a semiconductor device.
2. Description of the Background Art
Gallium nitride (GaN) type semiconductor materials are known to have superior properties such as a large bandgap that is approximately 3 times that of silicon (Si), a high breakdown electric field that is approximately 10 times that of silicon, and also a high saturation electron velocity. Research and development of the gallium nitride type semiconductor material for use in devices of high frequency and high power output in the field of radio communication are actively in progress, and has already come to the stage of practical use in devices directed to base stations for cellular phones. Moreover, the expectation of covering the two competing demands of high breakdown voltage and low loss, i.e. low ON-state resistance, that was difficult in conventional Si power devices, has attracted attention in the application to power devices. Since the logic value of the ON-resistance is inversely proportional to the breakdown electric field raised to the power of three, it may be possible to significantly reduce the ON-resistance in a power device based on gallium nitride to approximately 1/1000 that of a device based on silicon. Thus, group III nitride semiconductor crystals such as gallium nitride crystals stand out to be a promising material for optical devices such as an LED (Light Emitting Diode) and for electronic devices such as a transistor.
With regards to such group III nitride semiconductor crystals, Japanese Patent Laying-Open No. 2006-193348 (Patent Document 1) discloses a group III nitride semiconductor substrate having a specific resistance of at least 1×104 Ω·cm. In a fabrication method of the group III nitride semiconductor substrate, vapor phase growth such as HVPE (Hydride Vapor Phase Epitaxy), MOCVD (Metal Organic Chemical Vapor Deposition), or MBE (Molecular Beam Epitaxy) is employed to grow epitaxially a group III nitride semiconductor employing dichlorosilane (SiH2Cl2) and tetrachlorosilane (SiCl4: silicon tetrachloride) for the doping raw material of silicon (Si) qualified as the impurity element.
Japanese National Patent Publication No. 2007-519591 (Patent Document 2) discloses a monocrystalline gallium nitride that has an average density of less than 1×106 cm−2 and a dislocation density standard deviation ratio of less than 25%. In the fabrication method thereof, silane (SiH4) and the like are employed as the doping raw material of silicon.
Further, Japanese Patent Laying-Open No. 2005-101475 (Patent Document 3) discloses a III-V group nitride type semiconductor substrate characterized in that the carrier concentration distribution at the outermost surface of the substrate is substantially uniform. In a fabrication method thereof, dichlorosilane is employed as the doping raw material in HVPE.
When a group III nitride semiconductor crystal is to be grown by vapor phase growth such as HVPE, MOCVD or MBE, the concentration of n type impurities (dopant) in the group III nitride semiconductor crystal must be controlled in order to regulate the n type conductive property of the group III nitride semiconductor crystal. Silane and dichlorosilane employed as the doping gas in the aforementioned Patent Documents 1-3 to dope silicon qualified as the n type impurity may be decomposed before arriving at the underlying substrate and adhere to the reaction tube at the growth temperature of the group III nitride semiconductor crystal.
The doping gas employed in the aforementioned Patent Documents 1-3 may react with nitrogen gas or ammonia gas to generate an SixNy (silicon nitride) type compound (x and y are arbitrary integers).
It was difficult to control the concentration of silicon in the doping gas if the doping gas directed to supplying silicon was decomposed or caused reaction prior to arriving at the underlying substrate. As a result, the concentration of silicon taken into the group III nitride semiconductor crystal will vary, disallowing adjustment of the concentration of silicon taken into the group III nitride semiconductor crystal. Therefore, it was difficult to control the resistivity of the group III nitride semiconductor crystal with silicon as a dopant. Particularly, this problem was further noticeable when HVPE was employed since the decomposition of the doping gas and/or reaction with another gas was significant due to the entire heating of the reaction tube.
A possible consideration is to supply the doping gas at high rate for the purpose of preventing thermal decomposition of the doping gas or reaction with the raw material gas. However, the concentration distribution of the doping gas supplied to the underlying substrate will be degraded if the doping gas is supplied at high rate, leading to significant degradation in the in-plane distribution of the resistivity in the group III nitride semiconductor crystal.
Thus, there was a problem that the property such as the ON-resistance is degraded in the case where the resistivity is not regulated and a semiconductor device is fabricated employing a group III nitride semiconductor crystal substrate of high resistivity.
Further, in the case where a semiconductor device is fabricated employing a group III nitride semiconductor crystal substrate having poor resistivity in-plane distribution, the yield was degraded since the property such as the ON-resistance of the semiconductor device will vary.